30 



THE GLACIAL EPOCH. 



Emigration 

 caused by 

 the first 

 Glacial 

 Epoch. 



Differentia- 

 tion when 

 isolated in 

 different 



localities. 



Return to 

 the Polar 

 Easin. 



winters became longer and longer, until they met, and banished bird-life from the Arctic 

 Ocean for perhaps 100 thousand years. Fortunately the ancestors of the Charadriidse had 

 already acquired migratory habits, and were doubtless familiar with the four coast-lines 

 leading south ; but every year their summer breeding- grounds were pushed further south, 

 and their winter range had to be extended. In the struggle for existence some parties left 

 the shores altogether and established themselves on the banks of rivers and lakes, whilst 

 others even abandoned their aquatic habits and became residents of the prairies. It would 

 be carrying hypothesis too far to try and discover where the ancestors of each genus were 

 isolated and differentiated, though in many cases strong circumstantial evidence is not 

 wanting ; but when the glacial period passed away, and was followed by a warm period, we 

 may be sure that the long residence in various isolated localities, differing in climate, in 

 food, in the kind of protections against enemies, and in the character of the enemies 

 themselves, produced a variety of modifications, the accumulation of which was greatly 

 accelerated by the increased struggle for existence, caused by the emigration of the Arctic 

 birds to the already overcrowded districts further south ; the final result being that the one 

 species which left the Polar Basin at the beginning of the Prse-Pliocene glacial period, 

 returned when it was over, not one, but ten species, leaving at least ten others which never 

 returned at all. 



The disappearance of apparently perpetual winter at the North Pole, the melting away 

 of the ice in the Arctic Ocean, and the retreat of the glaciers almost to the tops of the 

 highest mountains, lessened for a time the fierceness of the struggle for existence, by opening 

 up once more the finest breeding-ground in the world for shore-feeding birds. The ten 

 species met in the Polar Basin, where their common ancestors had lived about half a million 

 years previously, and probably found its coasts and its islands as desirable a place of 

 residence as their ancestors had done — so much so, indeed, that only one out of the ten 

 failed to become circurnpolar. 



During the Miocene Period the conditions of life were probably so easy, that few of the 

 ten species were greatly modified, but what modification did take place became by constant 

 interbreeding the common property of the species. But whilst interbreeding was the 

 universal rule, those birds which neglected it and adopted the system of in and in breeding 

 producing degenerate offspring, which were soon stamped out in the struggle for existence, 

 cross breeding between any two of the ten species never occurred, or, if it did happen in 

 exceptional cases, produced no permanent result, the offspring being barren hybrids which 

 died out of themselves. 



But this paradise of waders was not eternal ; the same fate befell the ten species as 

 had befallen the one species (their common ancestor) five hundred thousand years or more 

 previously. Another glacial period came on ; the Arctic Ocean again became a sea of ice, 

 and the glaciers crept down again from their mountain homes and almost covered the land. 



The first important break in the continuity of the area of distribution of each species 

 was doubtless a barrier of ice, extending from the sea of ice at the North Pole, and coming 



